Switch-mode power converters are widely used in various power conversion applications, such as single-phase and three-phase power factor correction AC/DC rectifiers and DC/AC inverters. As the power levels of the converters are increased, multiple fast switching parallel-connected semiconductor power switches are typically employed to fulfill system application requirements. In some cases, the power switches form main boost switches for a boost converter, which are simultaneously turned on and off.
Two types of conventional power switches are insulated-gate bipolar transistors (IGBTs) and field-effect transistors (FETs). IGBTs exhibit lower conduction losses compared to FETs, but FETs exhibit faster switching capabilities than IGBTs. In many systems having high frequency and power requirements, IGBTs and FETs are both employed in the power supply powering the system. In effect, the IGBTs and FETs are combined to form a "single" switch in the power supply to take advantage of the FETs and IGBTs characteristics. In the process of operating the power supply, generally, the FETs are turned ON, i.e., conducting, first then the IGBTs are turned ON. Therefore, the FETs handle the switching "ON" loss while the IGBTs take care of the conduction loss. Thereafter, the IGBTs are turned OFF, i.e., not conducting, before the FETs are turned OFF. As a result, the IGBTs do not have switching power losses. For example, in a commercially available switch-mode rectifier (208Vin, -52V/220 A output), three IGBTs and three FETs are connected in parallel to form a single boost switch. The FETs are turned ON about tens of nanoseconds before the IGBTs and are turned OFF about a few hundreds of nanoseconds after the IGBTs are turned OFF. Thus, the IGBT experiences low switching power losses (due to the turn-ON and -OFF sequence of the IGBT) and, experiences low conduction losses (due to the nature of the switch, i.e., its lower ON voltage).
The IGBTs and FETs are typically formed in a switch sub-assembly and mounted on a common heat spreader to help dissipate the heat generated by the switches during operation of the power supply. For the switch-mode rectifier introduced above, two such heat spreaders are employed. A problem arises, however, after the switches have been assembled on the heat spreaders. Since both devices are commercially available with the same physical layout and packaging (e.g., an industry standard package TO-247 as provided by International Rectifier Corporation, El Segundo, Calif.) and with mounting clamps positioned over the switches on the heat spreader sub-assembly, it becomes difficult (if not impossible) to visually distinguish between the two kinds of switches. Furthermore, the switches are connected in parallel and are almost nearly switched simultaneously further exacerbating identification problem.
Under normal operating conditions and room temperature, a switch-mode rectifier with multiple parallel-connected switches may successfully pass its function test with switches in the improper location (FETs in place of IGBTs and vice versa). A rectifier with switches in the improper location may even pass its factory burn-in test under certain operating conditions. The failure to detect "swapped" IGBTs and FETs prior to delivery to the customer, though, seriously impacts the reliability of the rectifier. The most significant potential problem with respect to swapped switches is that at elevated operating temperatures, low input line voltage and at maximum rated output power, the unit may fail because the wrong switch (e.g., an IGBT) is absorbing all of the switching losses and/or conduction losses.
Accordingly, what is needed in the art is a system and method of distinguishing between the different types of switches that may otherwise be indistinguishable.